1. Field of the Invention
This invention relates to an inverting amplifier, and a crystal oscillator having the inverting amplifier. More particularly, the invention relates to those which are useful when applied as an alternating current inverting amplifier for an FET input, an MOS input or a CMOS input requiring a high input impedance, and a crystal oscillator having the AC inverting amplifier.
2. Description of the Related Art
FIG. 8 is a circuit diagram showing an inverting amplifier of a CMOS circuit according to the prior art. As shown in this drawing, this inverting amplifier is composed of a CMOS inverter comprising a P-channel MOS transistor 1 and an N-channel MOS transistor connected in series, and a self-bias resistance Rf connected between an input terminal In and an output terminal Out of the CMOS inverter.
FIG. 9 shows the input voltage and output voltage of the inverting amplifier, and a voltage distribution in the self-bias resistance Rf shown in FIG. 8. In this drawing, the input voltage is designated as Vin, the voltage gain of the CMOS inverter is designated as Av, and the output voltage is designated as Vo. Based on this drawing, a current If flowing through the self-bias resistance Rf is expressed asIf=(1+Av)Vin/RfThus, the input impedance Zin viewed from the input side is expressed asZin=Rf/(1+Av)Consequently, with the CMOS inverter having a great voltage gain Av, the effective input impedance has to be markedly decreased. In the case of the CMOS inverter used at 100 KHz or lower, for example, the voltage gain Av of the CMOS inverter in response to a small signal is 20 or more, so that the input impedance decreases to one-twentieth or less. This phenomenon is generally called the Miller effect.
Normally, the higher the input impedance of the amplifier, the better. If a decrease in the input impedance by the Miller effect is taken into consideration, therefore, the self-bias resistance Rf needs to take a very high value.
Japanese Patent Application Laid-Open No. 2004-096711 can be named as a prior art document concerned with the present invention.
The problems of the above-described earlier technology will be described by way of an inverting amplifier in a crystal oscillation circuit of 32 KHz as shown in FIG. 10. In this case, in order to ensure a large loop gain for the oscillator, the input impedance of the inverting amplifier is desired to be 10 MΩ or greater. Assuming that the gain Av of the inverting amplifier is 20, the value of the self-bias resistance Rf has to be set at 200 MΩ or greater in order to obtain an input impedance of 10 MΩ or more. If the value of the self-bias resistance Rf is set at 200 MΩ or higher, however, the following problems with practical use are posed:
1) If there is a slight current leakage or the like in an electrostatic protective circuit provided in an input or output terminal of the inverting amplifier, or in wiring connected to a device outside IC, such as a crystal resonator, the bias voltage of the inverting amplifier greatly varies, hindering an oscillatory action.2) When the inverting amplifier is formed into IC, it is not easy to build a large resistance, such as 200 MΩ, into IC.
Next, VCXO (voltage-controlled crystal oscillator) as shown in FIG. 11 will be described. If a self-bias resistance Rf is high in this VCXO, bias voltage is apt to change in response to changes in control voltage VC. Namely, DC bias is unstable, so that the changes in the control voltage VC are easily transmitted to the input of the oscillator, and their influence appears as duty changes in the output waveform. If the self-bias resistance Rf is rendered low to avoid the duty changes, the input impedance of the inverting amplifier naturally lowers, arousing a decrease in the loop gain of the oscillator.
The present invention has been accomplished in the light of the above-mentioned problems with the prior art. It is an object of the present invention to provide an inverting amplifier, which can decrease a self-bias resistance without lowering the input impedance of the inverting amplifier, and a crystal oscillator having the inverting amplifier.